1. Field of the Invention
The present invention relates to an optical amplifier/transmitter apparatus using an optical amplifier, and more particularly to a method and apparatus for preventing the occurrence of surge light at the time of signal input to an optical transmitter in such an optical amplifier/transmitter apparatus.
2. Description of the Related Art
In an optical amplifier/transmitter apparatus, there is often employed a system for amplifying an optical signal using an amplifier constructed from an erbium-doped fiber (EDF). Specifically in a transmitting section for transmitting an optical signal, an EDF amplifier is often used in an optical amplifier that is provided to amplify, prior to transmission, an optical signal fed from an optical transmitter that generates signal light in the form of modulated light on the basis of an input electrical signal.
In an EDF amplifier, a situation can occur where the input optical signal remains in the "space" state, i.e., the state of no input optical signal being present, for a prolonged time. In such a case, since pumping light continues to be supplied to the EDF, internal energy continues to increase. In this condition, when the input optical signal changes to "mark", i.e, when signal light is input, the output light level abruptly increases at the rising edge of the signal, producing light surge and causing the internal energy to drop, then settling to produce normally amplified optical signal output.
Such surge light is transmitted via a transmission path to the receiving end, and causes a detrimental effect on the optical receiver that receives the optical signal in the receiving section. That is, because of the excessively high light level, a photodiode (PD) or an avalanche photodiode (APD), constituting an optical detector in the optical receiver for detecting optical signals, is overloaded beyond its allowable level, and may eventually be damaged.